For a chemical ionization mass spectrometer which is interfaced with a gas chromatograph the peak resolution achieved by the gas chromatograph is often deteriorated to some degree because of an unavoidable sample build-up which results from insufficient source conductance. It is therefore highly desirable that after each chemical ionization operation the source conductance can be momentarily increased in order to prevent this sample build-up. Known chemical ionization mass spectrometers are not capable of performing this source venting operation in a repeatable and automatic manner.
It is very desirable and usually necessary for chemical analysis that sample material be treated in a plurality of chemical ionization modes by sequentially introducing each of a plurality of reagent gases into the ion source. Multiple chemical ionization modes are of value only if rapid and efficient venting of the chemical ionization source can be achieved after each chemical ionization operation mode. Known mass spectrometers are not capable of rapid switching from a first chemical ionization operation mode to a second chemical ionization mode in a cyclic repeatable and automatic manner.
For a mass spectrometer which is interfaced with a gas chromatograph it is also highly desirable that sample material which enters the ionization region from the gas chromatograph column be treated in both a chemical ionization mode and in an electron impact ionization mode within a period less than the residence time of the sample material within the ion source region. Numerous spectrometers have been designed and built which are capable of operation in both a chemical ionization mode and in an electron impact ionization mode within the same ionization region. However, each of these known mass spectrometers requires manual switching between a chemical ionization mode and an electron impact ionization mode. It is difficult, if not impossible, for known spectrometers to treat in both a chemical ionization mode and in an electron impact ionization mode each of a plurality of sample materials sequentially entering the ionization region from the gas chromatograph.
In most conventional quadrupole mass spectrometers, due to their inherent design, only ions of one polarity can be extracted and detected at any instant of time, in spite of the fact that both positive ions and negative ions are usually formed simultaneously under typical ionization conditions. Although some quadrupole mass spectrometers have been constructed allowing sequential detection of positive ions and negative ions by manually switching the voltage polarities of the ion source and the detector, this operation is achieved only with considerable waste of labor and sample materials.
In one recent design of quadrupole mass spectrometer it has been possible to extract positive ions and negative ions from the ion source region alternatively at high frequence by pulsing the voltage polarities of the ion source and lens electrodes. Although this type of operation achieves some improvements over the previous designs in mass spectrometers, the fact still remains that only ions of one polarity can be extracted at a time during which the ions of opposite polarity are lost without being ultilized.
For mass spectrometers employing magnetic sectors instead of quadrupole analyzers as their resolution devices, one design has been reported capable of extracting and analyzing both positive ions and negative ions simultaneously. This was achieved by employing two independent magnetic sectors, detectors and mass scanning circuits. However, due to the fact that two completely independent mass scanning circuits must be utilized, the obtained positive ion and negative ion mass spectra have no relationship to each other. This fact makes it very difficult to obtain accurate mass calibration for ions in one spectrum based on the spectrum containing ions of opposite polarities and known masses.
U.S. Pat. Nos. 2,999,157, 3,307,033, 3,405,263, 3,555,272, 3,573,453, 3,849,656, 4,005,291, and 4,066,894 disclose structure related to mass spectrometers. However, the structure shown in each of these patents has one or more of the limitations or disadvantages discussed above.
It is therefore an object of this invention to provide a mass spectrometer which may be interfaced with a gas chromatograph and in which the source conductance can be momentarily increased after each chemical ionization operation, to prevent sample build-up.
Thus, it is a further object of this invention to provide such a mass spectrometer which is capable of venting operations in a repeatable and automatic manner.
It is another object of this invention to provide a mass spectrometer which is capable of treatment of sample material in a plurality of chemical ionization modes by sequentially introducing each of a plurality of reagent gases into the ion source region.
Another object of this invention is to provide means by which any one of a plurality of operational cycles can be achieved.
Another object of this invention is to provide means for achieving rapid and efficient source venting by either manual or automatic operation.
It is another object of this invention to provide a quadrupole mass spectrometer which is capable of detecting both positive ions and negative ions truly simultaneously and also capable of facilitating the ion mass calibration for any one mass spectrum based on the information provided by the other spectrum.
Other objects and advantages of this invention reside in the construction of parts, the combination thereof, the method of manufacture, and the mode of operation, as will become more apparent from the following description.